This invention generally relates to membranes, and their use in separating a gas from a gas stream. In some specific embodiments, the invention relates to the preferential separation of hydrogen from synthesis gas mixtures, and related power generation systems.
Membranes are selectively permeable barriers that can be used to separate gases. One exemplary application for membranes is to separate gases in power generation, specifically integrated gasification combined cycle (IGCC) power plants. These plants generate electricity from carbonaceous fuel such as coal, petcoke, or biomass, through a series of steps, including gasification of the solid fuel to form a mixture of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), water vapor, and trace impurities. The mixture is commonly known as “synthesis gas” or “syngas”. Impurities are removed from the syngas mixture, through a series of clean-up operations. The cleaned gas is then combusted to produce electricity in a combined cycle.
IGCC plants offer advantages in efficiency because the clean-up of impurities is performed on high pressure gas streams before combustion. Membranes can be used in the IGCC clean-up process to separate the syngas into a fuel-rich stream that can be used to generate electricity, and a CO2-rich retentate stream to enable “carbon capture”. The use of a membrane for carbon capture can involve the selective permeation of CO2 through the membrane, separating it from the rest of the gas stream, or can involve the selective permeation of hydrogen, the primary fuel gas. In an ideal situation for some power generation systems, gas separation is carried out at high temperature and pressure, so as to minimize the necessity for compressing the CO2 prior to sequestration. In some cases, hydrogen-selectivity (as compared to CO2 selectivity) is a key parameter in a gas separation system. In addition, a water-gas-shift reactor is usually employed upstream of the membrane. The water-gas-shift reactor converts carbon monoxide into hydrogen and carbon dioxide, to maximize the overall thermal efficiency of the plant.
Many types of membrane structures are available for gas separation at relatively high temperatures. Most are based on metallic or ceramic materials. While dense metallic membranes are useful for some gas separation processes, they are also deficient in some respects. For example, the metals in such membranes are often intolerant of sulfur. Therefore, in separating gas mixtures which may include compounds like hydrogen sulfide (e.g., gas streams produced from sulfur containing feedstocks such as low rank coal, petcoke, or biomass), metallic membranes can suffer irreversible degradation.
Porous ceramic membranes can also be used for gas separation processes, provided the pore size can be sufficiently controlled to enable high selectivity. The International Union of Pure and Applied Chemistry (IUPAC) designates “microporous” as having pores less than about 2 nm; and “mesoporous” as having pores ranging from about 2 to 50 nm. In general, “microporous” membranes have the potential to show high selectivity for H2. Membranes with larger pores, for example “mesoporous” membranes, show limited H2 selectivity.
In the case of membranes with pores larger than about 2 nm, but smaller than the mean free path for a gas, the transport mechanism is predominantly Knudsen diffusion. Knudsen diffusion has a different temperature dependence than activated transport, with the flux decreasing with the square root of temperature, as the temperature increases. In membranes where transport is dominated by Knudsen diffusion, the ideal membrane selectivity for gases is the inverse square root of the ratio of their molecular masses. For example, Knudsen H2/CO2 selectivity is about 4.7.
In general, the formation of microporous membranes which have fine pores and high flux characteristics (i.e., flow capacity) can be difficult. As an example, since the flux through a membrane can decrease with decreasing pore size, it is often desirable to employ membrane layers which are as thin as possible. However, it can be difficult to manufacture thin, porous layers which have uniform pores, and which are also mechanically robust.
Silica-based membranes are well-known in the art for use in gas separation processes. The manufacture of the silica membranes is a relatively straightforward and economical process, and in some situations, the membranes are very effective for gas separation. State-of-the-art silica membranes often consist of a thin silica layer, on top of a supported, porous aluminum oxide layer, which provides mechanical strength. Unlike the metallic-based membranes, silica-based membranes are somewhat more tolerant to the presence of sulfur-based compounds.
However, there are considerable drawbacks associated with silica membranes. For example, in some cases, there is poor reproducibility in the fabrication process, which can result in large fluctuations in performance, e.g., separation properties. Moreover, under elevated temperature conditions, silica can be very sensitive to steam, which adversely affects the microstructure and gas separation performance of the membrane structure.
With some of these concerns in mind, new membranes and membrane structures, based in part on porous ceramic materials, would be welcome in the art. The membranes should exhibit good hydrogen selectivity. The membranes should also be relatively tolerant of harmful gases like hydrogen sulfide, and in general, should be suitable for use in corrosive atmospheres. Moreover, the membranes should be capable of economic fabrication, and should generally be compatible with a variety of power generation and gasification systems that utilize fossil fuels, or biomass.